1. Field of the Invention
The present invention relates to a transmission power control method and a transmission power control apparatus using the same, and more particularly to a transmission power control method of a mobile station in a CDMA (Code Division Multiple Access) system which performs multiple access using a spread spectrum technique in mobile communications, and a transmission power control apparatus using this method.
2. Description of Related Art
As is well-known, a CDMA system falls into two classes: a direct sequence (DS) system which spreads a conventionally modulated signal by using a high rate spreading code; and a frequency hopping (FH) system which resolves a symbol into elements called chips, and translates each chip into signals of different center frequencies at a high rate. Since the FH system is difficult to implement in the state of the art, the DS system is usually employed. Spread-spectrum radio systems differ from conventional communication systems for satellite data networks such as SCPC/FDMA (Single Channel Per Carrier/Frequency Division Multiple Access) systems, or TDMA (Time Division Multiple Access) systems in that the spread-spectrum radio systems transmit, at a transmitter side, a signal after modulating it by a common modulation, and then, performing a secondary modulation using a spreading code to widen its signal bandwidth. At a receiver side, on the other hand, the wideband received signal is despread to restore the narrow band signal, followed by a conventional demodulation processing. The despreading is performed by detecting correlation between the spread-spectrum sequence of the received signal and a spreading code sequence which is generated at the receiving station, and peculiar to the channel. The capacity in terms of the number of subscribers in a cell is determined by an SIR (Signal-to-Interference Ratio) needed to achieve a required error rate because a CDMA system uses the same frequency band for the subscribers.
Applying the CDMA system to the mobile communications presents a problem in that received signal levels at a base station from respective mobile stations vary significantly depending on the locations of the mobile stations, and this arises a "near-far problem", in which a large power signal masks a small power signal, thereby reducing the number of mobile stations communicatable at the same time. In other words, a communication quality of a channel in the CDMA system is degraded by signals from other communicators because the same frequency band is shared by a plurality of communicators and the signals from the other communicators become an interference.
FIG. 1 illustrates an interference state in a reverse channel (from mobile station to base station) due to other mobile stations. When a mobile station MS1 closer to a base station communicates with the base station BS1 simultaneously with faraway mobile stations MS2 and MS3, the received power of the base station BS1 from the near mobile station MS1 will be greater than that from the faraway mobile stations MS2 and MS3. As a result, the communications of the faraway mobile stations MS2 and MS3 with the base station BS1 will be degraded owing to the interference from the near mobile station MS1.
To overcome such a near-far problem, transmission power control has been introduced. The transmission power control regulates received power at a receiving station, or the SIR determined by the received power, such that the received power or the SIR becomes constant regardless of the locations of mobile stations, thereby achieving uniform communication quality in a service area. Thus, the signal power from the other communicators becomes the interference, and hence, the transmission power control is essential to prevent the signal power of the other communicators from growing much larger than the transmission power of the intended channel.
In particular, with regard to a reverse channel, each mobile station must control its transmission power such that the received power thereof at a base station becomes constant. In a CDMA system, in which the interference power is considered as white noise, the transmission power error is the most important factor in determining the capacity in terms of the number of subscribers in a cell. For example, a 1 dB transmission power error will reduce the capacity in terms of the number of the subscribers by about 30%. Since an FDD (Frequency Division Duplex) system is generally employed to achieve two way communications, a reverse channel and a forward channel (from base station to mobile station) are frequency divided, that is, transmitted carrier frequency and a received carrier frequency differ from each other. Thus, a closed loop transmission power control is generally used in the FDD system.
FIG. 2 illustrates a method of determining transmission power of a mobile station with respect to a thermal noise level. In FIG. 2, the reference character S designates desired received signal power at a base station, I designates interference power at the base station, I.sub.max designates maximum allowable interference power at the base station, which interference power depends on the system, and SNR designates a ratio of the desired received signal power S to the thermal noise power N at the base station. The transmission power control of a mobile station at the base station is performed as follows:
(1) The base station has set in advance a desired received signal level needed to achieve a satisfactory received quality of a signal from a mobile station.
(2) The base station measures the actual received signal level of the signal transmitted from the mobile station.
(3) The base station decides whether the actual received signal level of the signal from the mobile station is greater or smaller than the desired received signal level.
(4) The base station inserts a transmission power control bit corresponding to the decision result periodically into a frame of a forward signal. The transmission power control bit commands an increase or a decrease of the transmission power of the mobile station.
This closed loop transmission power control makes it possible to carry out a high accuracy transmission power control whose error is within a few dB.
On the other hand, open loop transmission power control is performed as follows: First, the level of a signal transmitted from the base station is measured; and second, signal power transmitted from the mobile station to the base station is reduced when the received signal level at the mobile station is large, or vice versa.
Although the closed loop transmission power control is more accurate than the open loop transmission power control, the closed loop control involves a time delay. This is because the base station measures the received signal level from the mobile station, evaluates the transmission power of the mobile station of a few transmission power control intervals later, and then sets the transmission power of the mobile station using a forward transmission power control bit. Thus, proper transmission power control will be difficult owing to the time delay involved in the closed loop control when a sudden change in transmission characteristics occurs.
Furthermore, to absorb instantaneous fluctuations due to Rayleigh fading, the transmission power control bit must be inserted into frames at a rate higher than the Doppler frequency. For example, assuming that communications are carried out using carriers of 2 GHz band, and a mobile station is moving at 60-70 km/h, the Doppler frequency becomes about 200 Hz, and the received level will fluctuate at the interval corresponding to this frequency. Accordingly, the transmission power control bit must be inserted into frames at every few millisecond period. Considering the frame efficiency, the number of transmission power control bits per transmission power control must be limited to one or two. Hence, quick changes in the transmission power cannot be achieved by this number of bits.
On the other hand, since there are many high buildings in urban areas, the propagation path of a mobile station may suddenly transfers from a shadow of a high building to a line of sight area, or vice versa. In such cases, the received signal level at the base station can vary by more than 30 dB. As long as the transmission power control in the reverse direction operates normally as described above, the base station's received powers of the signals transmitted from respective mobile stations become constant, and hence, uniform receive quality can be obtained. However, when a mobile station suddenly moves out of the shadow of a building to a line of sight area, the base station's received power of the signal transmitted from the mobile station suddenly increases, and this induces large interference to signals transmitted from other mobile stations.
FIG. 3 illustrates an example of received signal level changes at the base station. In such cases, the closed loop transmission power control cannot quickly reduce the transmission power because the closed loop control involves some delay of a certain time constant. This presents a problem in that large interference to other users takes place.
As described above, the reverse transmission power control at a mobile station generally employs a closed loop transmission power control in the conventional CDMA system because high accuracy power control is required. The closed loop transmission power control, however, includes some delay of a considerable time constant involved in the feedback loop, and hence, it cannot achieve quick changes in the transmission power.
On the other hand, since there are many high buildings in urban areas, the propagation path of a mobile station may suddenly move from a shadow of a high building to a line of sight area, or vice versa. In such cases, the received signal level at the base station will vary by a large amount. In particular, when a mobile station suddenly moves out of a shadow of a building to a line of sight area, the base station's received power of the signal transmitted from the mobile station suddenly increases, and this presents a serious problem in that it provides large interference to signals transmitted from other mobile stations.